The present invention relates to a process for preparing water-soluble phosphonomethyl ethers of cellulose (PMC) in an aqueous, alkaline medium which contains an organic solvent.
Cellulose ethers which carry an anion-active substituent are used in many processes and fields of application, as water-soluble thickening agents and/or as auxiliaries, e.g., as a protective colloid, flocculating agent, binder and adhesive, dispersing agent or film-forming agent. Among these fields of application are the making of paper, the production and application of building materials, the processing of fibers, the preparation of pharmaceuticals and cosmetics and the production of soaps and detergents, varnishes and paints, and food and luxury food. The best known commercial product of this type of cellulose ether is carboxymethyl cellulose (CMC) which is conventionally marketed in the form of its Na salt (NaCMC). Also gaining a certain importance are other carboxylalkyl ethers or sulfoalkyl ethers of cellulose and mixed ethers which contain further ether groups in addition to the monobasic anion-active substituent, normally carboxymethyl.
Organic phosphonic acids or their derivatives lend themselves particularly well to the purpose of linking polybasic anion-active substituents to the cellulose molecule. Cellulose reacting with phosphoric acid yields cellulose phosphates as a result of direct ester-linkage of the cellulose molecule to the phosphoric acid. By reaction with reactive, organic phosphonic acids or derivatives, e.g., halogeno-alkane phosphonic acids, cellulose ethers are produced which have a phosphonic acid group or one of its salts in their ether substituent, i.e., the cellulose molecule is linked to the phosphonic acid group via an oxygen bridge and an organic bridge. The cellulose phosphates can be easily hydrolized and are readily crosslinked. In comparison, the cellulose ethers are stable compounds in which crosslinking does not readily occur. These cellulose ethers which contain phosphonic acid groups are normally insoluble in water, since reaction with halogeno-alkane phosphonic acids resulting in more than very low degrees of substitution is difficult. However, water-soluble products have also been disclosed.
Both U.S. Pat. No. 2,979,374 and the paper "Phosphonomethylation of Cotton" by G. L. Drake, W. A. Reeves and J. D. Guthrie, published in Textile Research Journal, March 1959, pages 270 to 275, describe chemically modified, fibrous cellulose-based textile materials and a process of preparing these materials, in which certain of the hydroxyl groups of the polysaccharide are replaced by --O--CH.sub.2 --PO.sub.3 X.sub.2 groups, wherein X is either H or an alkali metal or NH.sub.4. The degree of modification ranges from 0.01 to 4 percent by weight. In the process for the preparation of these products, the fibrous starting material is reacted with an aqueous solution of an alkali metal salt of chloromethane phosphonic acid or another salt of this acid and an excess of alkali metal hydroxide. The products obtained may further be converted to the free acid derivative or to the corresponding ammonium salts. It is also possible to prepare water-soluble phosphonomethyl ethers of cellulose, if the degree of modification is chosen in such a way that at least 2 percent by weight of phosphorus are introduced. The starting materials mentioned are cotton fibers, cellulose hydrate fibers, aminized cotton fibers, carboxymethylated cotton fibers, paper and sulfoethylated cotton fibers. For the purpose of introducing the phosphonomethyl groups, either a metal salt of chloromethane phosphonic acid itself or of its acid chloride may be used; it is also possible to use the corresponding monoesters of diesters. The aqueous reaction solution contains from 10 to 30 percent by weight of reactive alkali metal hydroxide and from 1 to 30 percent by weight of the alkali metal salt of chloromethane phosphonic acid. The reaction is conducted by allowing the cotton to absorb from 125 to 200 percent by weight of the reaction solution and heating to a temperature from 75.degree. to 115.degree. C. for 5 to 30 minutes or heating to 140.degree. to 160.degree. C. for 2 to 10 minutes. The water-soluble products are obtained from reaction solutions containing from 20 to 25 percent by weight of NaOH and from 10 to 20 percent by weight of chloromethane phosphonic acid, at temperatures from 120.degree. to 150.degree. C. applied for 5 to 30 minutes. In the only example of preparing a water-soluble phosphonomethyl cellulose, cotton is treated with a solution consisting of 35.1 parts by weight of chloromethane phosphonic acid, 96.5 parts by weight of NaOH and 180 parts by weight of water, until it has absorbed 196 percent by weight of the solution. Heating to 140.degree. C. for 30 minutes is followed by washing in water which contains 20 percent by weight of ethanol. Depending on the kind of starting material used, the phosphorus content determined in the water-soluble product amounts to 2.12 or 2.4 percent by weight.
The crosslinked phosphonoalkyl celluloses according to German Offenlegungsschrift No. 26 00 930 (corresponding to U.S. Pat. No. 4,020,271) comprise basic molecules of a kind which, without crosslinking, would themselves be water-soluble, but which are rendered substantially water-insoluble by crosslinking with formaldehyde, epichlorohydrin, dichloroacetic acid, diepoxides or other known difunctional components. With respect to carrying out the etherification stage, reference is made to the previously mentioned U.S. Pat. No. 2,979,374.
German Offenlegungsschrift No. 14 93 227 (corresponding to U.S. Pat. No. 3,388,118) discloses a method of preparing modified polysaccharides, in which chloroacetamido methyl cellulose or a similar compound containing reactive halogen is first prepared, and this intermediate compound is then reacted with trialkyl phosphites. The products obtained are not cellulose ethers; they contain nitrogen and phosphorus in substituent groups. The first reaction stage is run in an aqueous solution of, for example, N-methylol chloroacetamide and the second reaction stage in a solution of the phosphite in dimethyl formamide (DMF). The reaction products are insoluble in water.
The methods for the preparation of ion exchangers according to German Auslegeschrift No. 20 05 407 (corresponding to United States Patent No. 3,634,394) or according to German Auslegeschrift No. 20 05 408 (corresponding to United States Patent No. 3,652,540) can also result in products which carry a phosphonomethyl group. In one method, for example, pearls of regenerated cellulose are reacted in toluene in the presence of benzethonium chloride [CH.sub.3 C(CH.sub.3).sub.2 -CH.sub.2 -C(CH.sub.3).sub.2 -C.sub.6 H.sub.4 -(O-CH.sub.2 -CH.sub.2).sub.2 -N.sup..sym. (CH.sub.3).sub.2 -CH.sub.2 -C.sub.6 H.sub.5, Cl.sup..crclbar. ] with a solution containing NaOH, NaBH.sub.4, water and chloromethane phosphonic acid for 16 hours at a temperature of 90.degree. C. The product obtained is insoluble in water.
In the known process for the preparation of water-soluble phosphonomethyl ethers of cellulose (PCM), temperatures exceeding 100.degree. C. must be applied, in order to activate the halogenomethane phosphonic acid or one of its salts, respectively, which is slower to react than other etherifying reagents (for example, monochloro acetic acid). Any water which is present in the reaction medium is largely evaporated in the course of the reaction and, as a result, the etherification reaction proceeds in an almost-dry mixture. Experience has shown that in such "dry" processes in which the alkali cellulose swells slightly, non-uniform etherified products with high proportions of unsubstituted chain portions are in most cases obtained. This applies in particular to products which have a low degree of substitution (e.g., a DS of less than 0.4)--as in the present case. If it is intended to prepare water-soluble cellulose ethers, this will lead to products which give turbid, fibrous solutions showing residues, even at degrees of substitution which are actually sufficient to impart solubility in water, and the products are, therefore, unsuitable for many fields of application. The high temperatures employed also have an unfavorable effect on the stability of the polymer chain, since an oxidative chain-disintegration may occur. As a result, the cellulose ethers thus prepared, when dissolved, will yield only low viscosities and are consequently hardly suitable, for example, for use as thickening agents.
The other previously disclosed processes do not lead to water-soluble phosphonomethyl celluloses or to ether groupings, respectively. Furthermore, they are also either conducted at a temperature above 100.degree. C. and with the aid of a "coupling component" or at a temperature below 100.degree. C., but with the addition of several auxiliary substances and at long reaction times.